Generally, open-circuit voltage is the voltage difference of electrical potentials between two terminals of a device when there is no external load connected. In the applications of gas discharge lamps, ballasts are used to limit the current flowing through the gas discharge lamps. Therefore, electrical ballasts have to provide sufficient open-circuit voltage to allow the gas discharge lamps to have enough energy to maintain the glow discharge state and transits from the glow discharge state to the arc discharge state.
FIG. 1 shows a block diagram of a ballast for gas discharge lamps according to the prior art. As shown in FIG. 1, a ballast includes a PFC (power factor corrector) 102, a DC/DC converter 104, an input filtering capacitor C1, and an inverter 106. The ballast is used to receive an AC input voltage Vin and provide the energy required to ignite the gas discharge lamp Lp1. The gas discharge lamp Lp1 is a high intensity discharge lamp (HID) lamp. The PFC 102 is used to rectify the AC input voltage Vin into a DC voltage with a corrected power factor for suppressing the harmonic noises in the input current. The input filtering capacitor C1 is connected to the output end of the PFC 102 for removing the noises and interferences of the DC voltage outputted from the PFC 102. The DC/DC converter 104 is connected to the input filtering capacitor C1 and is made up of a buck converter, which converts the DC voltage outputted from the PFC 102 into a lower DC voltage. The inverter 106 is connected to the output end of the DC/DC converter 104 for converting the output DC voltage of the DC/DC converter 104 into an AC voltage, thereby driving the gas discharge lamp Lp1. It is noteworthy that if the gas discharge lamp Lp1 is a DC lamp, the inverter 106 may be eliminated. The DC/DC converter 104 is used to control the operating state of the gas discharge lamp Lp1, thereby allowing the gas discharge lamp Lp1 to operate under the steady state with constant current mode or constant power mode.
FIG. 2 shows a block diagram of a ballast for gas discharge lamps according to the prior art, in which the detailed circuitry of the DC/DC converter 104 of FIG. 1. It should be noted that similar circuit elements are labeled with the same reference numeral. In FIG. 2, the DC/DC converter 104 is a buck converter and includes a switch Q1, a rectifying diode D1, an inductor L1, and a capacitor C2. The rectifying diode D1 is connected in parallel with the input filtering capacitor C1, and the capacitor C2 is connected in parallel with the rectifying diode D1. The inductor L1 is connected between the anode of the rectifying diode D1 and one end of the capacitor C2. The switch Q1 is driven by a driver (not shown) to conduct switching operations. The switch Q1 has a first terminal (or positive terminal), a second terminal (or negative terminal), and a third terminal (or control terminal). There are two possible ways to locate the switch Q1. The first way of locating the switch Q1 is to place the switch Q1 between the capacitor C1 and the cathode of the rectifying diode D1, i.e. the switch Q1 can be located at the high-voltage side of the DC/DC converter. However, such configuration requires an additional isolation device such as a photo coupler to drive the switch Q1. Hence, such configuration will increase the cost and make the switch Q1 to be driven in a difficult manner. The second way of locating the switch Q1 is to place the switch Q1 between the capacitor C1 and the anode of the rectifying diode D1, i.e. the switch Q1 can be located at the low-voltage side of the DC/DC converter, as shown in FIG. 2. Such configuration allows the second terminal of the switch Q1 to share the same ground point with the input filtering capacitor C1, thereby eliminating the isolation device and rendering the switch Q1 easy to be driven. Hence, the configuration of locating the switch Q1 is to place the switch Q1 between the capacitor C1 and the anode of the rectifying diode D1 is widely used. The operation of the DC/DC converter 104 is described as follows. The energy of the voltage on the input filtering capacitor C1 is transferred to the inductor L1 and the capacitor C2 by the switch operation of the switch Q1. The rectifying diode D1 provides a current path for the inductor L1 when the switch Q1 is turned off. The inductor L1 and the capacitor C2 form an output filter for removing the noises of the output voltage derived from the switching operation of the switch Q1. The output voltage of the DC/DC converter 104 is established on the capacitor C2.
Referring to FIGS. 1 and 8, in which FIG. 8 shows the waveform diagram of the lamp voltage and the lamp current of the ballast for gas discharge lamps. Before the gas discharge lamp Lp1 is ignited, the ballast of the gas discharge lamp Lp1 needs to provide an appropriate open-circuit voltage having a voltage level of 300V, for example, to supply sufficient energy for the gas discharge lamp Lp1 to transit from the glow discharge state to the arc discharge state. Hence, before the gas discharge lamp Lp1 is ignited, the lamp voltage Vlamp is the open-circuit voltage of the ballast. Also, before the gas discharge lamp Lp1 is ignited, the lamp current Ilamp flowing through the gas discharge lamp Lp1 is zero. After the gas discharge lamp Lp1 is ignited, the lamp current Ilamp is ascending and the impedance of the gas discharge lamp Lp1 is descending rapidly, thereby lowering the lamp voltage Vlamp of the gas discharge lamp Lp1. When the gas discharge lamp Lp1 enters the steady state, the impedance of the gas discharge lamp Lp1 is maintained at a stable value (the stable value is the impedance of the gas discharge lamp Lp1 which enters the steady state). In this way, the waveform of the lamp voltage Vlamp and the waveform of the lamp current Ilamp will be periodically fluctuating in positive half-cycles and negative half-cycles.
In prior art ballasts, a compensation circuit is required to substantially control the open-circuit voltage of the ballast within a predetermined range. Nonetheless, the circuit structure of the compensation circuit is quite complex and the efficiency of the ballast is reduced by using the compensation circuit. More disadvantageously, the cost of the ballast will increase due to the incorporation of the compensation circuit. The applicants propose a control device for controlling the open-circuit voltage of the ballast with a simplified circuit structure, thereby limiting open-circuit voltage of the ballast within a predetermined range.